Fen Management Handbook - Scottish Natural Heritage

7.2.1 Checking substrate fertility
The upper layers of soil are often the most fertile due to enrichment from surface
waters, agricultural activities or defecation by animals. Generally the underlying
substrate is more likely to be nutrient-poor, and thus capable of supporting species
rich vegetation.
Where aims for site management include reduction of substrate fertility and
encouragement of greater species diversity, samples of peat/substrate taken from
different depths should be analysed to assess whether surface layers are particularly
enriched in major plant nutrients. This can be assessed using a technique called
phytometry (see Section 10: Monitoring to Inform Fen Management) in which
seedlings of test plant species are grown in soil samples taken from different areas
of the site and different depths as appropriate. The tests can be extended with
and without additions of nitrogen and phosphorus. Comparisons of the biomass
produced in the test and control soils shows whether or not the layer of soil being
tested is already enriched with one or both of the major plant nutrients. This is often
considered to be a more reliable estimate of what is available to plants than analyses
of soil water and soil extracts. While such analyses are useful and informative,
they may not show how much is available to the plants, especially when critical
concentrations are close to the limits of detection.
An indication of substrate fertility can also be obtained from noting the plants that
grow in and around the margins of the proposed area for rejuvenation and comparing
them with information provided in, for example, the JNCC’s Common Standards
Monitoring Guidance
7.2.2 Removal of silt and other sediments
Heavy machinery may sink where the fen has become filled up with silt and other
soft sediments. Unconsolidated silts can be removed using a suction dredger,
but leaching of chemicals (such as oxidised iron) from the dredged silts can be a
pollution risk. It may also be difficult to establish wetland plants in un-consolidated
substrates as the plants are uprooted by wave-action.
Some sediment may be rich in heavy metals such as lead, iron and manganese which
may leach out. In the case of iron, oxidation may give rise to an obvious ochreous
colouration and deposit. Disposal of contaminated sediments to special waste
disposal sites is very costly, and requires relevant licences. In addition sediments are
often rich in phosphorus which will encourage unwanted plant growth.
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Works adjacent to watercourses should be carefully designed with
appropriate controls to prevent silt or other contaminants entering rivers
and streams.
A prior desk study of local sources of possible contaminants may suggest whether
there is any risk of contamination. As a precaution it is wise to analyse some of
the sediment prior to removal for concentrations of heavy metals such as iron,
manganese, lead, cadmium, copper and zinc.
Consult your local environmental protection agency (EA, SEPA, NIEA) at the
planning stage. They should be able to advise whether material will require analysis.